Wind turbines are used to convert wind energy to electrical energy in a sustainable and environmentally-friendly way. The use of wind turbines is gaining widespread acceptance in their role of providing alternative energy, and the installed capacity of wind turbine power generation has increased dramatically over the past decade.
As such, there are efforts being made to tighten up the electrical power grid requirements, in order to streamline the differing standards of different manufacturers, and to allow wind turbine generators to operate as conventional power generation systems, e.g. oil and gas plants, hydro-electric power plants, nuclear, etc. In various countries, the grid code standards have been revised and have called for more stringent requirements to be met.
Within such grid code standards are the requirements of operation in the presence of negative sequence voltages and operation within a wide frequency range. For example, some localities provide a standard of continuous operation within a 3% negative sequence voltage, i.e. the wind turbine should be able to withstand a maximum negative sequence voltage of 3% in the grid before connection. Also, most countries' grid codes call for operation of the power generation system within a range of frequencies around the operating fundamental frequency of the power grid.
Negative sequence voltages may arise from a system of voltage imbalances. In a power grid with a balanced sinusoidal system, the three line-neutral voltages are equal in magnitude and its phases are displaced from each other by 120 degrees. Any differences that exist in the three voltage magnitudes and/or a shift in the phase separation from 120 degrees is said to give rise to an unbalanced supply. Possible causes of an unbalanced system are perhaps unequal impedances of three-phase transmission and distribution lines, or many other possible reasons.
An unbalanced system can then be broken down into positive sequence voltages and negative sequence voltages. Positive sequence voltages are associated with a positively rotating field, while the negative sequence voltages are associated with a negatively rotating field. It is well known that an unbalanced supply voltage consisting both positive and negative sequence components will typically give rise to a DC link voltage variation at twice the line frequency if the grid current only has positive sequence components. The presence of ripple on the DC link voltage will affect control effectiveness on the machine side converter. In addition, it in turn may lead to asymmetry and distortion in the grid currents which might give rise to grid code incompliance issues.
With regard to frequency variation, the instantaneous frequency at any time may vary a few percent in either direction of the fundamental frequency, which is defined as either 50 Hz or 60 Hz depending on the country of installation. Usually in case of overproduction on a grid, the grid frequency rises and in case of underproduction, grid frequency falls. For countries with power grids operating at a fundamental frequency of 50 Hz, most grid codes require grid frequency operation within a range of 47-53 Hz. For countries with power grids operating at a fundamental frequency of 60 Hz most grid codes require grid frequency operation within a range 57-61.7 Hz. In this case, if the components inside WTG are not well designed, they could be damaged by operating at a frequency range other than desired one. On the other hand, if the current control is designed for operation at a certain frequency, it may not achieve a good performance when running at other frequencies.
Various implementations have been proposed to operate power systems to operate in the presence of negative sequence voltages, but solutions currently known are relatively complicated.
For example, U.S. Pat. No. 6,052,297 describes a power conversion apparatus to control the current of the power converter in a positive sequence synchronously rotating d-q frame (SRF) and a negative sequence synchronously d-q frame to account for the positive sequence voltages and the negative sequence voltages respectively. There is no teaching of accommodating a varying grid frequency. As taught in this publication, in order to achieve good performance in such a situation through current control, four PI controllers should be implemented to regulate positive sequence currents in positive sequence-SRF and negative sequence currents in negative sequence SRF separately. However, such a scheme would require four separate PI current controllers, in addition to feed forward and decoupling elements. This leads to undesired complexity. Besides complexity, sequence component extracting filters are required to be employed which undermine the overall regulator bandwidths and stability margins.
It is thus an object of the present invention to provide a method and a system for controlling a power converter in a wind turbine generator which is less complex than that which is known, as well as to satisfy one or more of the newly provided grid requirements as described above.